With the reduction in size and weight of electronic products, higher precision in patterning of printed circuit boards has been required. Since flexible film substrates can be bent, which enables three-dimensional wiring, they are suitable for reduction in size of electronic products. Therefore, the demand for flexible film substrates is increasing. With respect to tape automated bonding (TAB) techniques which are used for connecting ICs to liquid crystal display panels, by the roll-to-roll process with a relatively narrow polyimide film substrate, it is possible to obtain an excellent fine-pattern as a resin substrate. However, the development of micro-fabrication with a polyimide film substrate is substantially reaching its limits. In order to evaluate accuracy of a miniaturized pattern, an index represented by the line width and the space between lines, and an index represented by the position of the pattern on the substrate are used. With respect to the line width and the space between lines, further miniaturization may be possible. The latter index, i.e., dimensional change rate, relates to the alignment accuracy between a circuit board pattern and electrode pads when the circuit board and electronic components, such as ICs, are connected to each other, and as the pitch between electrode pads in an IC is further narrowed and the number of electrode pads in an IC is increased, it becomes difficult to meet the required accuracy. That is, when an IC having more than 400 to 1,000 electrode pads is connected to a circuit pattern, extremely low dimensional change rate is required to align all the electrode pads in an IC with fine electrode pads with a pitch of 60 μm or less, preferably 50 μm or less, of the circuit pattern.
In terms of dimensional change rate, it is, in particular, difficult to improve the fabrication of flexible film substrates. In the circuit board fabrication, heat treatment processes, such as drying and curing, and wet processes, such as etching and development, are performed, and the flexible film is repeatedly subjected to expansion and shrinkage. The hysteresis during the fabrication processes causes distortion of the circuit pattern on the substrate. In the case when a plurality of processes require alignment, if expansion and shrinkage occur during such processes, positioning error occurs between patterns formed. The distortion of the flexible film due to expansion and shrinkage more greatly affects a flexible printing circuit (FPC) in which a substrate with a relatively large area is treated. Additionally, positioning error is also caused by external forces, such as tension and torsion, and, in particular, when a thin substrate is used to increase flexibility, adequate care must be taken. Since the flexible film substrate expands and shrinks due to humidity and temperature even after the circuit formation, it is absolutely necessary to control temperature and humidity of the circuit board prior to the IC connection. Even when moisture proof packaging is used, the production cost is increased, it is difficult to achieve complete moisture proofing, and the guarantee period is limited. Under the circumstances, the upper limit of dimensional change rate of the circuit pattern formed on a flexible film has been considered to be approximately ±0.015 to ±0.030%, and it is becoming difficult to cope with further narrowing of the pitch and increase in the number of electrode pads in an IC.